Method of electroplating readily oxidizable metals

ABSTRACT

IN A METHOD OF ELECTROPLATING READILY OXIDIZABLE METALS AND ALLOYS, AN ARTICLE TO BE PLATED IS FIRST PICKED WITH A MIXTURE OF NITRIC ACID AND HYDROFLUORIC ACID OR A MIXTURE OF BORIC ACID AND HYDROFLUORIC ACID, IMMEDIATELY WASHED WITH AN ANHYROUS ORGANIC SOLVENT, DRIED BY AN INERT GAS OR UNDER VACUUM, DIPPED IN A PLATING BATH WITHOUT BEING EXPOSED TO THE ATMOSPHERE AND THEN AFTER A SHORT TIME ELECTROLYTICALLY PLATED.

United'states Patent Oflice 3,699,013 Patented Oct. 17, 1972 3,699,013 METHOD OF ELECTROPLATIN G READILY OXIDIZABLE METALS Akira Miyata, Hideyo Okubo, Chikayoshi Tomita, and Akio Suzuki, Kawasaki, Japan, assignors to Nippon Kokan Kabushiki Kaisha No Drawing. Filed Aug. 14, 1970, Ser. No. 63,938 Claims priority, application Japan, Aug. 26, 1969, 44/ 67,048 Int. Cl. C23b N US. Cl. 204-32 R 11 Claims ABSTRACT OF THE DISCLOSURE In a method of electroplating readily oxidizable metals and alloys, an article to be plated is first pickled with a mixture of nitric acid and hydrofluoric acid or a mixture of boric acid and hydrofluoric acid, immediately washed with an anhyrous organic solvent, dried by an inert gas or under vacuum, dipped in a plating bath without being exposed to the atmosphere and then after a short time electrolytically plated.

BACKGROUND OF THE INVENTION This invention relates to a method of electroplating readily oxidizable metals and alloys such as aluminum alloys, titanium alloys, chromium alloys, stainless steel and the like whose surfaces tend to be instantly oxidized in the air and are covered by dense oxide films. Accordingly according to conventional methods it has been difiicult to apply adhesive metal coatings on these metals by electroplating due to the unavoidable presence of such oxide films.

For this reason, with prior techniques, it has been impossible to form metal coatings by electroplating directly on the surface of metals which tend to be readily oxidized in air to form dense oxide films on their surfaces, for example aluminum alloys and magnesium alloys, so that it has been the practice to firstly subject these metals to a zincate treatment. This treatment is an alkaline plating treatment for depositing zinc; that is, when an article to be plated is immersed in a plating solution, the surface of the article slightly dissolves into the plating solution to deposit zinc on the surface. Then a desired metal is electroplated on the zinc film thus formed immediately or after plating a thin copper film thereon. Since this method utilizes the intermediate Zinc layer, the plated metal layer coexists with zinc, thus degrading the value of the plated layer of pure metal. Thus it has been long desired an efficient direct plating method.

When electroplating another metal on titanium alloys, stainless steel or chromium steel, these metals used to be washed with an acid to remove inactive surface films and then the metals were electrolyzed to activate their surfaces by using them as. cathodes or anodes. Alternatively, the metals are often etched to render coarse their surfaces for firm adhesiveness. After removal of the surface oxides, subsequent operations should be performed promptly in order to prevent the formation again of inactive films. Even with this method, in a short interval until the electroplatnig step is commenced, oxide films are formed momentarily on the surfaces of the metals and, although little, such intermediate oxide films between metal substrates and plated layers always hinder good adhesion between them. Accordingly, in order to form plated films of excellent quality, it is desirable to use a preliminary treatment which keeps elf oxide films.

Especially in the case of electroplating utilizing a molten salt, the plating bath is deteriorated by water or moisture which might be admixed therewith. It is necessary to SUMMARY OF THE INVENTION This invention contemplates the provision of a novel method of electroplating metals under conditions completely free from oxides on the surface of the article to be plated.

According to this invention, to improve adhesive property of the plated film, the workpiece is Washed with a mixture of nitric acid and hydrofluoric acid or a mixture of boric acid and hydrofluoric acid to completely dissolve the existing oxide film on the surface of the workpiece to expose an inner new metal surface. The hydrogen evolved during pickling is prevented from being absorbed in the base metal to deteriorate it by applying ultrasonic wave vibrations to the acids.

To remove the acids remaining on the surface of the workpiece before immersing it in the plating bath, the workpiece is usually washed with water after pickling. However, readily oxidizable metals are oxidized by the oxygen dissolved in the water during washing to form oxide films on the surface or to react with water to form films of hydroxide thus interferring with plating.

This invention is characterized in that after removing most of the acids remaining after pickling by washing with water for a short time or without washing with water after pickling, the workpiece is washed with a dehydrating organic solvent to remove acids and water from the surface of the workpiece. The solvent suitable for this purpose must mix well with acids and water to remove them perfectly from the metal surface, should not react with the metal of the workpiece, and can readily vapourize or should not adversely effect the plating operation even when such solvent is introduced into the plating bath without being removed from the metal surface.

As the result of exhaustive research we have found that ketones and alcohols such as ethylene glycol, methyl alcohol, ethyl alcohol and glycerin are most suitable solvents and that ethers and carboxylic acids are also efliective. To remove the organic solvent remaining on the surface of the workpiece after washing, an inert gas, with or without heating, is blasted against the surface to evaporate the remaining solvent, or the Washed workpiece is put in an evacuated vessel to evaporate the solvent. Where the organic solvent does not affect the plating bath when admixed therewith, the washed workpiece can be immediately immersed in the plating bath along with the organic solvent.

By this process, the surface of the metal workpiece is cleared enough and becomes free from any moisture or oxide film; however, in order to prevent prompt oxidation of the surface of the workpiece before electroplating the workpiece is handled in an inert gas such as nitrogen and argon and is then immersed in the plating bath without being exposed to the atmosphere. This is also one of the features of this invention.

Even when the workpiece is carefully handled in this manner the surface of the workpiece immersed in the plating bath is often covered by a thin oxide film caused by a small quantity of oxygen contained as an impurity in the inert gas which is utilized to intercept the atmosphere. Although thin, such an oxide film prevents good bonding of the film electroplated thereon, so that it is necessary to remove it before plating.

To accomplish this, according to this invention this thin oxide film is dissolved off while the workpiece is immersed in the plating bath. Especially as the electroplating bath comprising molten salt has a property to dissolve metal oxides, it is possible to dissolve off the metal oxides by immersing the workpiece in the plating bath for a suitable time without passing current through the bath. Dissolution of the oxides can be accelerated by passing alternating current between the plating bath and the workpiece thus removing a small quantity of the oxides on the surface thereof.

After the oxides have been completely removed from the surface of the workpiece by the preliminary treatment described above, the workpiece is connected to the negative pole of the source to commence an electroplating operation to obtain a firmly bonded plating film.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the following examples 1 through 8, aluminum films were electroplated on identical titanium alloys with an identical molten salt plating bath. Degreasing and pickling treatments, the composition of the plating bath, conditions of the plating-process, and test of the bonding strength of the plated layers were the same in all of these examples. However, the conditions of treatment between pickling and immersion of the workpiece in the plating bath were varied.

Example 1 Sample: titanium alloy (Ti-6 Al-4V) 0. 8 mm. thick x 25 mm. wide x 100 mm. long Treatment: after degreasing and pickling the sample was washed with acetone and then electroplated without drying.

The sample was degreased by dipping it in petroleum benzene and acetone and then immersed in a mixture of 450 cc. of concentrated nitric acid and 66 cc. of hydrofluoric acid for 1 and one half minutes at room temperature.

After pickling, the sample was immediately dipped in acetone and washed while being moved slowly for 10 seconds to completely remove acids remaining on the sample.

After washing with acetone and while still wetted with acetone the sample was sealed in nitrogen gas atmosphere and quickly transferred to an electrolytic plating bath comprising a molten aluminum salt.

The composition of the plated bath consisted of 60 mol percent of aluminum chloride, 25 mol percent of sodium chloride and mol percent of potassium chloride and maintained at a temperature of 160 C.

Two pure aluminum plates having substantially the same dimensions as the sample were placed in parallel with the sample on the opposite sides thereof and an alternating current of the intensity of 2 a./dm. was passed between the aluminum plates and sample for 3 minutes to clear the surface of the sample.

Thereafter, aluminum films were plated on the surface of the sample for 35 minutes at a current density of 2 a./dm. with the sample acting as the cathode and the aluminum plates as anode electrodes.

After removal from the plating bath, the plated sample was thoroughly washed with a large quantity of aqueduct water (hereinafter, merely designated water): To assure perfect washing and rinsing ultrasonic wave vibrations were applied to the water during washing. After washing the sample was dried in hot air.

Although plated layers of poor adhesion formed by prior art methods peel off during the ultrasonic wave washing, it was found that plated layers formed as above described did not peel off, which shows that the bonding strength of the plated layers is very high.

Example 2 Sample: Titanium alloy identical to that of Example 1.

Treatment: after degreasing, pickling and washing with water, the sample was rinsed with ethyl alcohol and then immediately plated without drying.

Again the sample was degreased by diping it in petroleum benzene and acetone, and then pickled in the mixture of nitric acid and hydrofluoric acid for one and one half minutes.

The sample was washed with water for 10 seconds while being moved slowly and rinsed with ethyl alcohol for 10 seconds. The sample was then plated with aluminum films under the same conditions of electroplating and with the identical plating bath as in Example 1. The plated sample was Washed with water under supersonic vizrations in the same manner as in Example 1.

Again it was found that the plated layers did not peel off when subjected to the ultrasonic vibrations, thus showing strong bonding.

Example 3 Sample: titanium alloy identical to that of Example 1.

Treatment: after degreasing, pickling, and washing with water, the sample was rinsed with ethyl alcohol. After drying in nitrogen the sample was electroplated.

In the same maner as in Example 1 the sample was degreased with petroleum benzene and acetone, and then pickled with the mixture of nitric acid and hydrofluoric acid for 1.5 minutes.

The sample was then washed with water for 10 seconds followed by a rinsing with ethyl alcohol for 10 seconds.

After removal from ethyl alcohol, nitrogen gas was blasted against the sample at normal temperature for 30 seconds to evaporate off the ethyl alcohol.

Thereafter the sample was plated With aluminum under the same conditions of electroplating and with the identical molten salt bath as in Example 1.

Again the plated aluminum layers did not peel off by washing with water under supersonic waves, thus showing strong bonding.

Example 4 Sample: titanium alloy identical to that of Example 1.

Treatment: after degeasing, pickling, and washing with water, the sample was rinsed with methyl alcohol and then plated without drying.

Sample: titanium alloy of the same dimensions and composition as that of Example 1.

Treatment: after degreasing, pickling and washing with water, the sample was rinsed with a mixture of ethyl alcohol and glycerine and then immediately plated without drying.

The sample was again degreased by immersing it in petroleum benzene and acetone and then pickled with the mixture of nitric acid and hydrofluoric acid in the same manner as in Example 1.

Thereafter the sample was washed with water for 10 seconds, rinsed with a 1:1 mixture of glycerine and ethyl alcohol for 10 seconds and then immediately dipped in a plating bath. Aluminum plating was performed with the identical molten salt bath and under the same plating conditions as in Example 1.

Again the plated aluminum layers did not peel oif during washing with water under supersonic waves, thus showing good bonding.

Example 6 Sample: titanium alloy identical to that of Example 1.

Treatment: after degreasing, pickling and washing with water, the sample was rinsed with methyl alcohol, dried in nitrogen and then plated.

The sample was degreased with petroleum benzene and acetone and then pickled for 1.5 minutes with the mixture of nitric acid and hydrofluoric acid, in the same manner as in Example 1.

Then the sample was washed with water for seconds, rinsed with methyl alcohol for 10 seconds, and then nitrogen gas was blasted against the sample at normal temperature for 30 seconds to evaporate off the methyl alcohol remaining on the surface of the sample.

Thereafter, the sample was plated with aluminum under the same condition of electroplating and with the identical molten salt bath as in Example 1.

The plated aluminum layer did not peel off during Washing with water under supersonic waves thus showing strong bonding.

Example 7 Sample: titanium alloy identical to that of Example 1. Treatment: After degreasing, pickling and washing with water, the sample was rinsed with ethyl ether and then immediately plated.

The sample Was degreased with petroleum benzene and acetone pick-led for 1.5 minutes with the mixture of nitric acid and hydrofluoric acid.

Thereafter the sample was washed with water for 10 seconds, rinsed with ethyl ether for minutes, and then immediately immersed in the plating bath to be plated with aluminum under the same conditions and with a molten salt bath identical to that of Example 1.

The plated aluminum layer peeled off only at one point during washing with water under supersonic waves but other points showed good bonding.

Example 8 Sample: titanium alloy identical to that of Example 1.

Treatment: after degreasing, pickling and washing with water, the sample was rinsed with acetic acid anhydride and then immediately plated without drying.

In the same manner as in Example 1, the sample was degreased by dipping it in petroleum benzene and acetone and then pickled for 1.5 minutes with the mixture of nitric acid and hydrofluoric acid.

After washing with water for 10 seconds, the sample was rinsed with acetic acid anhydride for 20 seconds and then immediately dipped in the plating bath to be plated with aluminum film under the same conditions and with an identical molten salt bath as that of Example 1.

When washed with water under supersonic waves in the same manner as in Example 1, the plated aluminum layer peeled off at several small areas but the remaining portions showed good bonding.

From the foregoing examples, it was noted that titanium alloy plates can be plated with aluminum layers having a high bonding force by the method of this invention. In order to confirm that the same good results can also be obtained for different titanium alloys, the following examples are shown to compare the results of this invention as aplied to a titanium alloy having a composition of Ti, 8 Al, 1 Mo and 1 V which has been considered diflicult to plate with aluminum platings diflicult to peel off.

Example 9 Sample: titanium alloy of the composition of Ti, 6 Al and 4 V of the dimension of 0.8 mm. thick x 100 mm. wide x 150 mm. long.

Treatment: after degreasing and pickling the sample was rinsed with acetone and then immediately plated without drying.

The sample was degreased by washing it with petroleum benzene, and then immersing it in acetone for one minute.

Thereafter the sample was dipped in a mixture of 460 cc. of 60% nitric acid and 36 cc. of 46% hydrofluoric acid at room temperature for 30 seconds.

After pickling, the sample was immediately dipped in acetone and moved slowly to completely remove the acids remaining on the sample. Thereafter the sample was immediately immersed in a plating bath while still wetted with acetone.

The plating bath used was a molten salt bath having a composition of 60 mol percent of aluminum chloride anhydride, 25 mol percent of sodium chloride and 15 mol percent of potassium chloride maintained at a temperature of 160 C.

Activation of the sample surface by alternating current in the plating bath was carried out under the same conditions as in Example 1.

Aluminum was then plated with the sample connected to act as the cathode and aluminum plates as the anode at a current density of about 0.4 "a./dm. for the first five minutes and at a current density of 2 a./dm. for a subsequent 35 minutes.

The reason for using low current density during the initial period and increasing it later to the normal current density is to improve the adhesion of the plated film to corners and peripheries where the current density becomes higher.

In the same manner as in Example 1 the plated sample was washed with water under supersonic waves and then dried with hot air.

Aluminum plating formed in this manner had a thickness of about 10 microns and purely white appearance and did not peel off under ultrasonic waves.

The aluminum plating was also subjected to a peening treatment by blasting minute beads of glass With compressed air against the plated sample. When subjected to such a peening treatment, plated layers of poor bonding generally tend to peel oif from the substrate and swell. But the plated aluminum layer plated by the method of this invention did not swell and maintained its silver white beautiful metallic luster appearance even when subjected to the peening operation. This means that the plated layers of this invention have an excellent adhesive property.

Example 10 Sample: titanium alloy of the composition of Ti, 8 Al, 1 Mo and 1 V and having dimensions of 1 mm. thick, 25 mm. wide and mm. long.

Treatment: after degreasing and pickling the sample was washed with acetone and them immediately plated without drying.

Although it is known that plated films readily peel off a titanium alloy of this composition, the sample was treated in the same manner as in Example 9 and it was found that the bonding properly was similarly excellent. The following Examples 11 through 14 show examples wherein aluminum layers were electroplated on aluminum substrates in a molten bath of aluminum salt.

Example 11 Sample: diecast aluminum alloy (Federal, A 13, Si 12%) having dimensions of 10 mm. thick, 75 mm. wide and 150 mm. long.

The sample was washed with petroleum benzene and then degreased by dipping it in acetone for one minute. Then the sample was dipped in a mixture of 360 cc. of 63% nitric acid and 40 cc. of 46% hydrofluoric acid for 30 seconds at room temperature.

After pickling, the sample was immediately dipped in acetone and washed with it while being moved slowly for 5 seconds to completely remove the acids remaining on the sample. While still wetted with acetone, the sample was dipped in a plating bath consisting of 60 mol percent of aluminum chloride anhydride, 25 mol percent of sodium chloride and 15 mol percent of potassium chloride and maintained at a temperature of 160 C. The surface of the sample was activated in the same manner as in Example 1. Then the sample was plated with aluminum by connecting the sample to act as the cathode and the aluminum plates as the anode. For the first minutes the current density was maintained at about 0.4 a./dm. while in a subsequent stage the current density was increased to 2 a./dm. This current was passed for 90 minutes.

After plating the sample was taken out from the plating bath, thoroughly washed with a large quantity of running water and then rinsed with water under supersonic vibrations. After rinsing the sample was dried with hot air.

The resulted aluminum plating had a thickness of about 25 microns and purely white appearance. The plated layer was adhered uniformly and did not peel off by washing under supersonic vibrations.

After being subjected to a peening treatment, the surface became beautiful silver white and no tendency of swelling and peeling off was noted, thus showing strong bonding.

Example 12 Sample: plated aluminum of Example 11. Treatment: after degreasing and pickling the sample was washed with acetone and then plated without drying.

In this example, an additional aluminum plating was applied on the aluminum layer plated on the diecast aluminum alloy.

After degreasing the sample by dipping it in petroleum benzene and acetone, the sample was again dipped for seconds in an identical mixture of nitric acid and hydrofiuoric acid to that employed in Example 11.

Thereafter, the sample was washed with acetone to completely remove the remaining acids, and then immersed at once in the plating bath while wetted with acetone to apply an additional aluminum layer by the same plating steps as in Example 11.

By successively forming two aluminum layers in this manner, it is possible to obtain a beautiful aluminum layer having a thickness of 50 microns which is bonded strongly to the substrates.

Example 13 Sample: aluminum foil Treatment: after degreasing and pickling, the sample was rinsed with acetone and then immediately plated with out drying.

By the repeated application of aluminum layers as in Example 12, an'aluminum film of extreme thickness was obtained.

For example, five plating operations were repeatedly applied for an aluminum foil of 100 microns thick under the same conditions as in Example 11 and the thickness of the resulted aluminum film was determined to be 120 microns by measuring it with a micrometer.

Example 14 Sample: diecast aluminum alloy, (Federal A 380 containing 8.5% of Si and 3.5% of copper) having dimensions of 10 mm. thick, 75 mm. wide and 150 mm. long.

Treatment: after degreasing and pickling, the sample was washed with acetone and then immediately plated without drying.

The sample was treated in the same manner as in Example 11. After electroplating, a pure aluminum coating was obtained having a thickness of 25 microns, strong bonding strength and beautiful appearance.

The following Examples 15 through 17 show the application of this invention wherein fasteners of chromium steel and titanium alloy were electroplated with aluminum by using a molten aluminum salt bath.

8 Example 15 Sample: a fastener of chromium steel Treatment: after degreasing and pickling, the sample was washed twice with acetone and then immediately plated without drying.

25 fasteners made of A181 HH steel (containing 5% of chromium and having dimensions of 6.5 mm. outer diameter and 21 mm. long) were contained in a wire net bag of Saran (Vinylidene Chloride, registered trade name) and dipped in acetone to degrease them. Then the fasteners contained in the Saran bag were immersed in a mixture of 4% boric acid and hydrofluoric acid contained in a polyethylene beaker. Ultrasonic waves of 29 kc., 300 w. were applied through the bottom of the beaker to wash the fasteners for 2 minutes at room temperature.

After pickling, fasteners contained in the Saran bag were then transferred into acetone and washed thoroughly by moving the bag vertically. A similar washing operation was repeated in fresh acetone.

After draining oif acetone the washed fasteners were placed in the plating bath while their surfaces were still wetted by acetone. The plating bath consisted of 60 mol percent of aluminum chloride anhydride and 40 mol percent of sodium chloride, and was maintained at a temperature of 160 C.

The operation of the plating apparatus was started by passing an alternating current for 2 minutes between aluminum plates and fasteners. Thereafter, the flow of the alternating current was interrupted and immediately thereafter a DC current of 40 a. was passed with the fasteners connected to act as the cathodes and aluminum plates as the anodes. After 45 minutes the flow of DC current was interrupted to stop the plating operation.

The plated fasteners were taken out of the plating bath, thoroughly washed with a large quantity of running water to remove remaining molten salt and then rinsed with distilled water for one minute under ultrasonic wave vibrations. Finally the plated fasteners were dried under heat by a centrifugal drier.

The plated fasteners were then subjected to a peening treatment for 5 minutes with glass beads (having diameters of about microns) to obtain silver white and beautiful aluminum films of 10 to 12 microns thick and having a metallic luster.

When subjected to the preliminary treatments merely involving pickling, washing with water and drying, these fasteners of H 11 steel could not be provided with aluminum platings of high bonding force. Such platings peeled off readily, similar to aluminum platings on common steel stocks.

Example 16 Sample: fasteners of chromuim steel Treatment: after degreasing and pickling, the sample was washed twice with acetone, dried with hot nitrogen gas and then plated.

300 fasteners of SISI 8740 steel containing 0.35 to 0.65% of chromium and having an outer diameter of 6.5 mm. and a length of 31 mm. were put in a cage of stainless steel and boiled in a 10% potassium orthosilicate solution for 30 minutes to remove oils and fats. After washing with water and confirming that degreasing was complete, as evidenced by wetting of water, the fasteners were put into a polyethylene beaker which was then filled with 50% hydrochloric acid incorporated with an inhibitor until fasteners were completely immersed. Then ultarsonic vibrations of 29 kc., 300 w. were applied through the bottom of the beaker at room temperature for 10 minutes.

Then, the fasteners in the acid solution were transferred into a basket of stainless steel and then dipped in acetone so as to remove remaining acid by moving the basket. This washing operation was repeated with fresh acetone.

Fasteners washed with acetone were then dried by a hot nitrogen atmosphere and then placed in an aluminum plating bath consisting of 60 mol percent of aluminum chloride anhydride and 40 mol percent of sodium chloride and maintained at a temperature of 160 C.

The plating apparatus was started by passing an alternating current of 50 a. for two minutes between the fasteners to be plated and aluminum plates.

Then the flow of the alternating current was interrupted for a DC of 72 a. was passed for 60 minutes by connecting the fasteners to the negative side and the aluminum plates to the positive side of a DC source.

Subsequent to the interruption of the DC current the plated fasteners were taken out of the plating bath, thoroughly washed with water for 10 minutes, and then rinsed with distilled water under ultrasonic vibrations to completely remove chloride remaining on the fasteners. Thereafter the plated fasteners were heat dried in a centrifugal drier.

Then the plated fasteners were subjected to a peening treatment with glass beads of the diameter of about 100 microns in a peening barrel. By the process steps described above, there were obtained aluminum plated layers of silver white appearance having a thickness of 12.5 microns and metallic luster.

Fasteners plated with aluminum as above described were treated with a chromium sulphate solution, then treated with a chromate treating solution, washed with Water and dried.

The platings had an apperance of yellow to orange colour and improved weather proofness and resistance to brine.

Example 17 Sample: fasteners of titanium alloy Treatment: after degreasing and pickling, the sample was washed twice with acetone, dried with nitrogen gas and then plated.

25 fasteners of titanium alloy (Ti, 6 Al and 4 V) having an outer diameter of 7.5 mm. and a length of 30 mm. and adapted for use in airplanes were put in a wire net of Saran and then dipped in acetone to degrease them by oscillating in the vertical direction. Degreased fasteners contained in Saran wire net were dipped in the mixture of nitric acid and hydrofluoric acid for 5 minutes with application of ultrasonic wave vibrations.

Pickled fasteners contained in the Saran wire net were dipped in acetone and the remaining acids were removed by oscillating the wire net in the vertical direction. The washing operation was repeated with fresh acetone.

Fasteners were removed from acetone together with the Saran wire net and then transferred into a glass cylinder through which nitrogen gas was passed to evaporate off acetone remaining on the surfaces of the fasteners. After drying, the wire net was brought immediately above the plating device, then the bottom of the wire net was opened to drop fasteners into the plating bath. The molten plating bath used comprised 60 mol percent of aluminum chloride (anhydrous) and 40 mol percent of sodium chloride. It was maintained at a temperature of 160 C.

The plating device was started by passing an alternating current of 50 a. between the fasteners and aluminum plates for 5 minutes. After interrupting the flow of alternating current, a DC current of a. was passed for 5 minutes by connecting the aluminum plates to the positive pole and the fasteners to the negative pole of the DC source and thereafter current was increased to 40 a. This current was passed for 45 minutes. Thereafter the operation of the plating device was stopped and plated fasteners were taken out of the plating device, washed with a large quantity of water and rinsed twice with distilled water under supersonic wave vibrations.

It was noted that plated films did not peel off by the washing with water under supersonic wave, thus showing strong bonding.

After washing with water, the fasteners were dried by a centrifugal drier and then subjected to a peening treatment utilizing glass beads to obtain beautiful silver white aluminum platings of 11 to 12 microns thick which did not peel off or swell.

The following Examples 18 to 20 show the application of this invention to aluminum plating on stainless steel substrates by using a molten aluminum salt bath.

Example 18 Sample: stainless steel A151 304 containing 18% of chromium and 10% of nickel and having dimensions of 1.0 mm. thick, 25 mm. wide and mm. long.

Treatment: after degreasing, pickling and washing with water, the sample was dried in vacuum and then plated.

After treatment with an alkaline degreasing liquid, the sample was washed with a mixture of 450 cc. of concentrated nitric acid and 66 cc. of 49% hydrofluoric acid for 2 minutes .at room temperature followed by washing with a running water for 20 seconds. Immediately thereafter, the sample was put into an evacuated vessel to remove water remaining on the surface of the sample. Then the vacuum was broken by the admission of nitrogen gas to dip the sample in the plating bath without exposing it to the atmosphere. The plating bath consisted of 60 mol. percent of an hydrous aluminum chloride, 25 mol percent of sodium chloride and 15 mol percent of potassium chloride. It was maintained at a temperature of C.

Two pure aluminum rods were placed in the aluminum bath in parallel with the sample on the opposite side thereof and an alternating current of one a. (50 cycles) was passed for three minutes between aluminum rods and the sample to clean the surface of the sample.

Then a direct current was passed for 30 minutes at a current density of 2 a./dm. by connecting the sample to the negative pole and the aluminum rods to the positive pole of the source.

After plating, the sample was taken out of the plating bath, immediately washed with a large quantity of water, rinsed with water under ultrasonic wave vibrations and finally dried with hot air.

The aluminum layer plated on the surface of the stainless steel sample had a thickness of about 8 microns and pure white smooth surface.

No peel off of the plated layer caused by washing with water under application of the ultrasonic sound waves was noted, which showed that the bonding of the plated layer was satisfactory.

Example 19 Sample: stainless steel Treatment: after degreasing and pickling, the sample was washed with water, rinsed with methyl alcohol, dried in vacuum and then plated.

The degreasing and pickling steps for the sample were the same as those of Example 18. After pickling, the sample was washed with water for 10 seconds, washed with methyl alcohol for 20 seconds for the purpose of accelerating drying and dried in an evacuated vessel. Then the vacuum was broken by introducing nitrogen into the vessel, and then the sample was dipped in the plating bath While preventing it from being exposed to the atmosphere.

The composition of the plating bath and the conditions of electrolysis were the same as those of Example 18.

The aluminum layer plated in this manner on the surface of the stainless steel sample had a thickness of about 8 microns and strong bonding force similar to that of Example 18.

1 1 Example 20 Sample: stainless steel Treatment: after degreasing, pickling and washing with water, the sample was rinsed with acetone, dried in vacuum and then plated.

The degreasings and pickling steps for the sample were carried out in the same manner as in Example 18.

After pickling, the sample was washed with water for 6 seconds, rinsed with acetone for 18 seconds, and then immediately put in an evacuated vessel to remove the acetone remaining on the surface of the sample. The vacuum was then broken by admitting nitrogen gas into the vessel and dipping the vessel in a plating bath while preventing the sample from being exposed to the atmosphere.

The composition of the plating bath and the conditions of electrolysis were the same as those of Example 18.

Again the aluminum layer plated on the stainless steel sample showed excellent adhesion.

Following Example 21 shows the application of this invention to the electroplating of an aluminum layer on a substrate of metallic chromium by using a molten bath of aluminum salt.

Example 21 Sample: metallic chromium (steel stock, 1.0 mm. thick,

25 mm. wide and 100 mm. long, and plated with chromium to a thickness of microns) Treatment: after degreasing, pickling and washing with water, rinsing with acetone, the sample was dried in vacuum and then plated.

The sample was dipped in an aqueous solution of 50% caustic potassium for 4 minutes at a temperature of 80 C. After washing with water, the sample was pickled with the same mixture of nitric acid and hydrofluoric acid as that used in Example 18, for 15 seconds at room temperature. Then the sample was washed with water for 30 seconds, rinsed with acetone to remove moisture and then plated with aluminum under the same conditions and with the identical plating bath as those of Example 18.

Again the resulted plated layer showed no tendency to peel ofi by washing with water under application of intense ultrasonic vibrations, thus exhibiting excellent adhesion.

As above described, this invention provides a novel method of electroplating metals or alloys readily oxidizable in the air under clean conditions of the surface quite free from oxides. For this reason, it is possible to directly electroplate dense metal layers without using any intermediate layer, on metals which have been considered diflicult to be applied with plated layers by the prior techniques or even when plated layers are applied these layers tend to peel off readily. For this reason, this invention finds many applications in wide fiields with satisfactory results.

For example, since high strength aluminum alloys are incorporated with alloying components having a tendency of affecting anticorrosion property of the alloys, products utilizing these alloys are defective in their corrosion proofness. However, when these products are electroplated with pure aluminum layers by the method of this invention it is possible to impart sufficient corrosion proof characteristics properly to their surface layers. Moreover, since the aluminum plated products can be further subjected to anode treatment, it is possible to render them highly corrosion resistent. Plates are now cladded with corrosion resistant aluminm layers but it has been difficult to improve the corrosion resistant property of the products of complicated configurations. In the airplane industry in addition to aluminum alloys, titanium alloys and chromium steel of high tensile strength are also used in a large quantity, and the corrosion proof property of these metals can also be increased by plating them with aluminum 12 coatings by the method of this invention. Especially such coatings are efiicient when they are used in contact with aluminum alloys. In this manner, this invention is expected to be used extensively in the airplane industry.

While the invention has been described in terms of a number of preferred embodiments, it will be clear that many changes and modifications may be made within the true scope and spirit of this invention as defined in the appended claims.

What is claimed is:

1. A method of electroplating a substrate with aluminum, said substrate being selected from the group consisting of a metal and a metal alloy which is readily oxidized in the air to form a dense oxidized film on the surface thereof, comprising the sequential operations of (a) pickling said substrate with a mixture of nitric acid and hydrofluoric acid or a mixture of boric acid and hydrofluoric acid,

(b) washing said pickled substrate with an anhydrous solvent selected from the group consisting of an alcohol, an ether, a ketone, a carboxylic acid and a carboxylic acid anhydride,

(0) after operation (b), directly contacting said washed substrate with an inert gas, and

((1) without exposing the substrate obtained in (c) to the atmosphere and directly after operation (c), forming a coating of aluminum thereon by placing said substrate in a molten salt bath comprising aluminum chloride and electrolytically depositing aluminum on the substrate using an aluminum plate as the anode and the substrate as the cathode.

2. The method of claim 1, wherein (d), the substrate obtained in (c) is placed in said bath and alternating current is applied thereto prior to electroplating said sub strate with aluminum.

3. The method of claim 1, wherein said solvent is a ketone.

4. The method of claim 1, wherein said solvent is a carboxylic acid.

5. The method of claim 1, wherein said substrate is titamum.

6. The method of claim 1, wherein said substrate is a titanium alloy.

7. The method of claim 1, wherein said substrate is aluminum.

8. The method of claim 1, wherein said substrate is an aluminum alloy.

9.1 The method of claim 1, wherein said substrate is a stee 10. The method of claim 1 wherein said solvent is evaporated from said washed substrate of (h) prior to 11. The method of claim 10, wherein said evaporation is performed under vacuum.

References Cited UNITED STATES PATENTS 2,898,230 8/1959 Bullolf 204--33 2,825,682 3/1958 Missel et al. 204-32 R UX 3,066,084 '1 l/1962 Osterrnan, Jr., et al. 204-34 UX 3,476,073 9/1969 Miyata et al. 20439 OTHER REFERENCES Domnikov, Larissa: Electroplating on Titanium, Metal Finishing, March 1962, pp. 59-62.

JOHN H. MACK, Primary Examiner W. I. SOLOMON, Assistant Examiner U.S. Cl. X.R. 204-33, 34, 39 

